A Candidate Recurrent Nova in M31: X-ray and UV short-term variability with Swift

ATel #5633; M. Henze, J.-U. Ness (both ESA/ESAC), M. F. Bode, M. J. Darnley, S. C. Williams (all LJMU), A. W. Shafter (SDSU)
on 6 Dec 2013; 14:16 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

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A detailed analysis of the X-ray and ultraviolet (UV) detections of a candidate recurrent nova (RN) in M31 (see ATel #5627) revealed significant variability within the Swift observation. Five separate snapshots were taken for the 6-ks observation and we determined the X-ray count rates and UV magnitudes for each of them. The table below lists the measurements.

 
Time           X-ray          UV          Exp_time             
[ks]        [10-2 ct/s]      [mag]           [s] 
 0.0          2.7±0.7        >20.23          720 
 5.6          1.1±0.3      19.6±0.2         1270 
11.4          2.8±0.7        >20.15          730 
17.2          1.3±0.3      19.7±0.2         1680 
34.5             <1.2      19.2±0.1         1660 

The starting times of the snapshots are given in kiloseconds after the start of the observation at 2013-12-03.03 UT. The exposure times in seconds are based on the good time intervals of the XRT event file. All UV magnitudes are in the uvw2 filter (112-264 nm) of the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) and have not been corrected for extinction. The X-ray counts rates were computed for the entire XRT energy range but are effectively restricted to below 1 keV due to the supersoft nature of the source. All count rate and magnitude errors are 1σ uncertainties and the upper limits are the 3σ confidence limits for a non-detection. The X-ray count rate estimates take into account separate exposure maps and dead time corrections. We checked the XRT instrument map and there are no bad columns at the source position in any of the snapshots. Therefore, the X-ray variability is unlikely to be caused by instrumental effects. The UV variability of the nova is clearly visible in the five separate images when compared to a nearby star.

X-ray variability during the early stages of the supersoft X-ray source (SSS) phase has been observed in several Galactic novae (e.g. RS Oph; see Osborne et al. 2011, ApJ 727, 124). This effect could be interpreted as caused by variable absorption during the emergence of the SSS from the expanding ejecta and/or optically thick wind. In this case, the SSS might have become visible only shortly before our observation on day six after the optical discovery (ATel #5607), which is nevertheless unprecedentedly fast. Short-term X-ray variability of various kinds has been observed in the M31 nova sample (see Henze et al., 2013, arXiv:1312.1241).

The table shows that the X-ray and UV variability appear to be anti-correlated. This could indicate a reprocessing of the SSS flux in the ejected material and would be consistent with the scenario of the SSS gradually emerging after the outburst. A similar anti-correlation between UV and hard X-rays had been seen in the Galactic nova V458 Vul (Ness et al., 2009, ApJ, 137, 4160).

We thank the Swift Team for carrying out the ToO observations.

Correction: The UV magnitude reported in ATel #5627 also refers to the uvw2 filter and not the uvw1 filter. The given wavelength range was correct. We apologise for any inconvenience caused by this typo.